The invention relates to a caster assembly for a traveling body, and specifically to a caster assembly having an automatic swivel lock/unlock feature that prevents the caster from swiveling when the caster is a trailing caster on the traveling body and allows the caster to swivel when the caster is a leading caster on the traveling body. The caster mechanism automatically switches between locked and freely swiveling modes solely in response to a reversal in the direction of the traveling body. Thus, the leading casters are always freely swiveling and the trailing casters are always restrained from swiveling.
Conventionally, a traveling body, such as a child's stroller, is fitted with wheels at each of the four corners of the body. If the orientations of all four wheels are fixed with respect to the stroller it cannot easily be turned or moved along a curved path. Generally, each wheel's axis of rotation should intersect the center of motion of the traveling body when the body moves along a curved path.
A wheel arrangement known for pivoting a wheel, particularly wheels used on traveling bodies such as strollers, is a caster wheel. A caster wheel can be defined as a wheel in which the wheel's axle (which defines its axis of rotation) is mounted to a wheel mount, which is pivotable about a vertical pivot axis, with the wheel's axle offset horizontally from the wheel mount's pivot axis. This geometry provides a stable arrangement in which the wheel's axle will tend to trail the pivot axis when a horizontal motive force is applied to the mount, the axle tending to align perpendicularly to the direction of motion (and thus to intersect the center of motion).
A conventional arrangement is therefore to couple the leading wheels (with respect to the direction of motion of the stroller) to the stroller so that their axis of rotation can be changed relative to the stroller (i.e., the leading wheels can pivot about a vertical axis). When the front wheels are pivotable and the trailing wheels are fixed, the stroller can be readily steered by an operator along a fixed path.
The arrangement of two permanently pivotable wheels (casters) and two permanently fixed wheels is unsuitable if the stroller is to be movable in both forward and reverse directions, i.e., a particular wheel may be leading when the stroller is moved in a forward direction and trailing when the stroller is moved in a reverse direction. The steering characteristics of the stroller are unfavorable if the leading wheels are fixed and the rear wheels are pivotable. In this configuration the stroller is susceptible to "boat" steering, in which the rear of the stroller must be moved a large amount for a relatively small change in the direction of motion. The handling characteristics are also poor if all wheels are pivotable--the stroller will tend to roll down hill. So if the travel surface is not level (perpendicular to the direction of motion) the stroller will veer off its intended course. It is therefore desirable to provide each wheel with a mechanism to allow a wheel to pivot when the wheel is leading, and to fix the wheel when it is trailing.
Several such mechanisms have been proposed that require manual actuation by the user. U.S. Pat. No. 3,636,586 to Bollinger et al. and U.S. Pat. No. 3,751,758 to Higbee et al. disclose manually operated caster locks that inhibit swiveling of the wheel by the downward rotation of a locking member to a position along the sides of the wheel. Once the lock is manually engaged and the wheel swivels to the predetermined position, the locking members rotate down to engage the sides of the wheel to prevent swivel.
The caster lock disclosed in U.S. Pat. No. 4,779,879 to Kassai, uses a manually controlled operator knob that is connected to the front and rear casters by a wire. The operator can select the front or rear wheels to be locked in a nonswiveling configuration.
Another manually operated caster lock is disclosed in U.S. Pat. No. 4,684,148 to Glaser. This design uses a locking member that is attached to the frame and pivots down to engage a recess in the caster to prevent swiveling of the wheel.
None of the patents described above disclose an "automatic" swivel locking mechanism--they all require the operator to take some action to engage the swivel lock.
U.S. Pat. No. 2,583,858 to Kostolecki discloses an automatic swivel locking caster in which a locking mechanism holds the caster wheel in a predetermined position with a spring-loaded ball and mating recess. This mechanism will prevent swiveling of the wheel when the ball and recess come into alignment and will allow swivel when the rotational force on the swivel mechanism (about its vertical axis of rotation) becomes great enough to overcome the force of the compression spring and dislodge the ball from the recess.
This design suffers from the drawback that it requires a lateral force to dislodge the ball from the recess to permit swiveling of the wheel. If the direction of travel of the traveling body is simply reversed (such as by stopping the body, and then moving it in an opposite direction) the necessary rotational force would not be present and the wheels would incorrectly remain in the swivel locked configuration. Furthermore, in some applications, such as light weight strollers or suitcases, it would be undesirable to require the application of a significant lateral force.
Another automatic caster wheel assembly is disclosed in U.S. Pat. No. 4,494,272 to Morita. This design uses an inner wheel supporting frame pivotally mounted in a second swiveling frame. This second frame is rotatably mounted on the mounting base. When the direction of the vehicle changes from a forward to a reverse direction, a pawl engages the wheel, preventing rotation of the wheel and the frictional force of the ground on the wheel pivots the inner frame to disengage the swivel lock. When the vehicle changes back to its original direction, the wheel swivels towards its locking position and the camming surface of the swivel lock mechanism depresses a spring urged engaging pin until the grooved section of the locking mechanism comes into original alignment with the engaging pin once again preventing swivel. This mechanism is somewhat complex mechanically.